Fluid streams derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of acid gases, for example carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), carbon disulfide (CS2), hydrogen cyanide (HCN), carbonyl sulfide (COS), or mercaptans as impurities. These fluid streams may be gas, hydrocarbon gases from shale pyrolysis, synthesis gas, and the like or liquids such as liquefied petroleum gas (LPG) and natural gas liquids (NGL).
Various compositions and processes for removal of acid gasses are known and described in the literature. It is well-known to treat gaseous mixtures with aqueous amine solutions to remove these acidic gases. Typically, the aqueous amine solution contacts the gaseous mixture comprising the acidic gases counter currently at low temperature or high pressure in an absorber tower. The aqueous amine solution commonly contains an alkanolamine such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA), or 2-(2-aminoethoxy)ethanol (sometimes referred to as diglycolamine or DGA).
In some cases, an accelerator is used in combination with the alkanolamines, for example piperazine and MDEA as disclosed in U.S. Pat. Nos. 4,336,233; 4,997,630; and 6,337,059, all of which are incorporated by reference herein in their entirety. Alternatively, EP 0134948 discloses mixing an acid with select alkaline materials such as MDEA, to provide enhanced acid gas removal.
Tertiary amines, such as 3-dimethylamino-1, 2-propanediol (DMAPD), have been shown to be effective at removing CO2 from gaseous mixtures, see U.S. Pat. No. 5,736,115. Further, in specific processes, e.g., the Girbotol Process, tertiary amines have been shown effective in removal of H2S, but show decreased capacity at elevated temperatures, for examples see “Organic Amines-Girbotal Process”, Bottoms, R. R., The Science of Petroleum, volume 3, Oxford University Press, 1938, pp 1810-1815.
Particularly important is the removal of sulfur based contaminants including hydrogen sulfide from fluid streams from oil and gas wells due to the highly noxious nature of these gases. Certain attempts at selective removal of sulfur based compounds have been made.
Tertiary alkanolamines such as MDEA are inherently selective for hydrogen sulfide over CO2. Because of increasingly more stringent specifications towards hydrogen sulfide and sulfur dioxide emissions, there is a need for aqueous alkanolamine formulations capable of removing hydrogen sulfide selectively over CO2 along with treating the gas to a very low level of H2S (i.e. 10 ppmv).
EP 01,134,948 discloses the use of low pKa acid additives (lower than 7) to enhance the selective removal of hydrogen sulfide. The technology aims at altering vapor liquid equilibrium characteristics of the alkanolamine solvent in order to achieve lower amount of hydrogen sulfide in the treated gas. U.S. Pat. No. 4,892,674 discloses the use of severely hindered alkanolamine salts as an additive for an MDEA gas treating solvent in order to enhance the selective removal of hydrogen sulfide over CO2 compared to MDEA alone. This technology is a combination of the use of severely sterically hindered amine and low pKa acid additives to MDEA based solvents. US 2010/0288125 discloses the use of phosphonic acid additives in order to enhance hydrogen sulfide selective removal. The premise of this disclosure is that phosphonic acid additives are superior to known sulfuric and phosphoric acid additives.
The hydrogen sulfide selectivity achieved with aqueous tertiary alkanolamine solutions such as water and MDEA mixtures is limited by the hydrolysis reaction of carbon dioxide and water. It is therefore desirable to replace some or all of the water in such a mixture with a solvent that is not reactive towards CO2. The premise of this adjustment is that hydrogen sulfide selectivity will increase by minimizing CO2 hydrolysis.
U.S. Pat. No. 4,545,965 discloses a process using tertiary amines with organic solvents in substantially anhydrous (<2 wt % water) solutions for selective hydrogen sulfide removal. The hybrid mixtures disclosed demonstrate improved selectivity compared to aqueous alkanolamine solutions. This process relies on substantially low water concentrations (<2 wt %), solvent with low dielectric constant, and amines with low pKa's.
U.S. Pat. No. 4,085,192 discloses a process for removal of hydrogen sulfide using an aqueous mixtures of alkanolamine and sulfolane. The preferred amines are diisopropanolamine (DIPA) and methyldiethanolamine. This invention suffers from the limited selectivity of DIPA for H2S over CO2 since DIPA is a secondary amine. Whereas, MDEA based hybrid formulations display low acid gas carrying capacity.
U.S. Pat. No. 4,405,585 discloses a process and formulation for selective hydrogen sulfide removal using aqueous blends of sterically hindered amines and physical solvent (preferred solvent is sulfolane). This process relies on sterically hindered amines having a low dielectric constant. In addition, the commercial usefulness of severely sterically hindered alkanolamine is somewhat limited by their difficult preparation as exemplified by patent publication WO 2005/081778 A2.
U.S. Pat. No. 5,705,090 discloses hybrid formulations for selective hydrogen sulfide removal using aqueous blends of polyethylene glycols and methyldiethanolamine. MDEA based hybrid formulations display low acid gas carrying capacity. In addition, polyethylene glycols display a rather low dielectric constant.
The Amisol process (Kohl & Nielsen, p 1231) uses aqueous blends of methanol and amines for selective hydrogen sulfide removal. The amines include diisopropylamine (DIPA) and diethylamine which display low vapor pressure and low dielectric constant as well as diethanolamine (DEA) which is not selective for H2S over CO2.
WO 86/05474 discloses hybrid solvents for selective hydrogen sulfide removal. Amines include tertiary amines and sterically hindered amines. Physical solvents include glycols, glycol esters, glycol ethers, and N-methylpyrrolidone. These solutions are anhydrous (<5 wt % water).
Selectivity achieved with aqueous tertiary alkanolamine solutions such as water and MDEA mixtures is limited by the base catalyzed hydrolysis reaction of carbon dioxide and water. It is know that hydrogen sulfide selectivity may be improved by replacing some or all of the water in such a mixture with a physical solvent that does not react with carbon dioxide. While reducing water improves selectivity, it has also been shown to considerably diminish the capacity of the solvent for carrying acid gases. This limitation has prevented hybrid solvents from seeing widespread application in selective hydrogen sulfide applications. We anticipate that a hybrid solvent formulation that overcomes the capacity limitation will offer superior performance to aqueous tertiary alkanolamines.
While the above processed solutions are effective, they each have limitations which detract from selective extraction of sulfur gases.
Therefore, there is a need for formulations and processes incorporating a minimal amount of water and capable of carrying high concentration of acid gas at low to medium acid gas partial pressures.